Optical networks that employ passive architectures are often referred to as Passive Optical Networks (PONs). Such networks use some form of passive component such as an optical star coupler or a static wavelength router and thus do not have any active switching elements. A primary advantage of a PON is its reliability, ease of maintenance and the fact that the field-deployed network does not need to be powered. Accordingly, PONs are often used as access networks by cable TV and telecommunications providers for the purpose of distributing their services from their facility to the customer premises (e.g., a home or business).
FIG. 1 shows the architecture of a PON in its most generalized form. The PON 100 includes a hub 102, remote nodes 104 that are deployed in the field, and network interface units (NIUs) 106. The hub 102, remote nodes 104 and NIUs 106 are in communication with one another over optical fiber links. If the PON 100 is a telecommunications network, hub 102 is a central office. If the PON 100 is a CATV network, hub 102 is generally called a head end. The NIUs 106 may be terminal equipment located on the customer premises or they may serve multiple customers, in which case the NIUs 106 simply provide another level in the network hierarchy below the remote nodes.
FIG. 2 shows a portion of a conventional PON 200 that is sometimes employed in a cable TV system. PON 200 includes a head end 202 having a driver amplifier 204, a 1×N splitter 206 and a high power optical amplifier 208 that is coupled to one of the outputs of splitter 206. As explained below, additional optical amplifiers (not shown) may be coupled to the remaining outputs of the splitter 206 as the capacity of the network is increased. Finally, the output of the high power optical optical amplifier 208 is coupled to an input of a second 1×N splitter 210. Each output from the splitter 210 is coupled to a remote node 212, which may be located in the field or on customer premises.
In operation, driver amplifier 204 typically receives an optical signal with about 1-4 mw of power and provides an amplified optical signal with about 100 mw of power to the 1×N splitter 206. If 1×N splitter 206 is an 1×8 splitter, high power optical amplifier 208 receives an optical signal with about 10-12 mw of power, after losses in the splitter are taken into account. In turn, the high power optical amplifier 208 provides an optical signal to the second splitter 210.
Driver amplifier 204 and high power amplifier 208 are generally rare-earth doped fiber amplifiers that use rare-earth ions as the active element. The ions are doped in a fiber core and pumped optically to provide gain. While many different rare-earth ions can be used to provide gain in different parts of the spectrum, erbium-doped fiber amplifiers (EDFAs) have proven to be particularly attractive because they are operable in the spectral region where optical loss in the fiber is minimal. Because of the electronic structure of the erbium ion, EDFAs can be pumped with optical energy at a wavelength of 980 nm or 1480 nm. Driver amplifier 204 is typically supplied with pump energy at 980 nm to achieve a lower noise figure and high power amplifier 208 is generally supplied with pump energy at 1480 nm to achieve higher output power (at the expense of an increase in noise relative to the driver amplifier 204).
One advantage of the arrangement shown in FIG. 2 is its scalability. That is, as demand for service grows, additional high power optical amplifiers can be added to the remaining unused outputs of the 1×N splitter 206. The driver amplifier 204 and splitter 206 are generally located in a common chassis and the high power optical amplifiers are modules that plug into the chassis. Thus, increasing capacity simply requires the provision of additional modules into the chassis. Moreover, capacity can be increased in this manner without any interruption in service. This arrangement is also highly reliable and requires minimal upfront cost. One disadvantage of this arrangement, however, is that as demand continues to grow, the increasing number of high power amplifier modules that are required makes the head end increasingly expensive.
Accordingly, it would be desirable to provide a scalable passive optical network whose capacity can be increased in a relatively inexpensive manner.